I'm not sure. She did not answer your question.
"To state" just means "to say."
Your teacher did not say how you should state your opinion. I think that an opinion should always use words that show it is an opinion. <---- For example, this statement itself is my opinion, so I began it with the words "I think."
Even though I began it with the words "I think," it is still a statement. It is not a statement of fact, but it is a statement of opinion.
Your teacher did not say where to state your opinion within the essay. Your teacher said it should be "the topic sentence." A topic sentence states the main idea of a paragraph, and it is the first sentence of the paragraph. Normally, in an essay, every paragraph begins with its own topic sentence.
If the main point of your essay is to state your opinion and then support it, then it could be the very first sentence of the whole essay--the topic sentence of the introduction. After you have proven your point, you might state your opinion again, in a different way, as the topic sentence of the conclusion.